The automaticity of the heart, pathology and pharmacology

Abstract

The automaticity of the heart is the ability of its cells to depolarize spontaneously, reaching the threshold potential and propagating action potential. The whole process happens in the electrical conduction system of the heart. The cardiac action potential is generated by the movement of ions through the transmembrane ion channels in the cardiac cells. An abnormal condition in which the heartbeat is irregular, too fast or too slow, is called a cardiac arrhythmia. A heart rate that is too fast – above 100 beats per minute in adults – is called tachycardia and a heart rate that is too slow – below 60 beats per minute – is called bradycardia. Antiarrhythmic agents are a group of pharmaceuticals that are used to suppress abnormal rhythms of the heart. According to the Singh Vaughan Williams classification of antiarrhythmic agents there are five main classes. Class I agents interfere with the sodium (Na+) channels. Class II agents are antisympathetic nervous system agents. Most agents in this class are beta blockers. Class III agents affect potassium (K+) efflux. Class IV agents affect calcium channels and the AV node. Class V agents work by other or unknown mechanisms. The antiarrhythmic medications have different mechanisms of action and many different individu-al drugs within these classes. Although the goal of drug therapy is to prevent arrhythmia, nearly every anti-arrhythmic drug has the potential to act as a pro-arrhythmic, and thus must be carefully selected and used under medical supervision.